* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The Electrochemical Gradient - Advanced
Survey
Document related concepts
Cytoplasmic streaming wikipedia , lookup
Lipid bilayer wikipedia , lookup
Node of Ranvier wikipedia , lookup
Mechanosensitive channels wikipedia , lookup
Cell encapsulation wikipedia , lookup
Chemical synapse wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
SNARE (protein) wikipedia , lookup
Signal transduction wikipedia , lookup
Cytokinesis wikipedia , lookup
P-type ATPase wikipedia , lookup
Action potential wikipedia , lookup
List of types of proteins wikipedia , lookup
Endomembrane system wikipedia , lookup
Cell membrane wikipedia , lookup
Transcript
The Electrochemical Gradient Advanced Douglas Wilkin, Ph.D. Niamh Gray-Wilson Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-source, collaborative, and web-based compilation model, CK-12 pioneers and promotes the creation and distribution of high-quality, adaptive online textbooks that can be mixed, modified and printed (i.e., the FlexBook® textbooks). Copyright © 2015 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/about/ terms-of-use. Printed: December 21, 2015 AUTHORS Douglas Wilkin, Ph.D. Niamh Gray-Wilson www.ck12.org C HAPTER Chapter 1. The Electrochemical Gradient - Advanced 1 The Electrochemical Gradient - Advanced • Describe the electrochemical gradient. Do you really have electricity flowing through your body? Yes you do. These electrical signals allow information to flow through the nervous system extremely rapidly. And it all starts with the formation of an electrochemical gradient. The Electrochemical Gradient The active transport of ions across the cell membrane causes an electrical gradient to build up across this membrane. The number of positively charged ions outside the cell is usually greater than the number of positively charged ions in the cytosol. This results in a relatively negative charge on the inside of the membrane, and a positive charge on the outside. This difference in charges causes a voltage to exist across the membrane. Voltage is electrical potential energy that is caused by a separation of opposite charges, in this case across the membrane. The voltage across a membrane is the membrane potential. Membrane potential is very important for the conduction of electrical impulses along nerve cells. The membrane potential of a cell at rest is known as its resting potential, and is discussed below. A non-excited nerve cell is an example of a cell at rest. Because of the ion gradient, there are less positive ions inside the cell, the inside of the cell is negative compared to outside the cell. This resulting membrane potential favors the movement of positively charged ions (cations) into the cell, and the movement of negative ions (anions) out of the cell. So, there are two forces that drive the diffusion of ions across the plasma membrane—a chemical force (the ions’ concentration gradient), and an electrical force 1 www.ck12.org (the effect of the membrane potential on the ions’ movement). These two forces working together are called an electrochemical gradient. The electrochemical gradient determines the direction an ion moves by diffusion or active transport across a membrane. In mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force, due to both the proton gradient and voltage gradient across the membrane. This potential energy is used for the synthesis of ATP by oxidative phosphorylation. The Resting Potential In order to maintain the membrane potential, cells maintain a low concentration of sodium ions (Na+ ) and high levels of potassium ions (K+ ) within the cell (intracellular). The sodium-potassium pump moves three Na+ ions out of the cell and brings two K+ ions into the cell. This essentially removes one positive charge from the intracellular space. The resulting membrane potential is known as the resting potential. FIGURE 1.1 This diagram shows how ions maintain the membrane potential. The sodium-potassium pump is shown in the membrane, transporting three Na+ ions (green) out of the cell and bringing two K+ ions (blue) into the cell. The Ion Gradient The electrochemical potential across a membrane determines the tendency of an ion to cross the membrane. The membrane may be that of a cell or organelle or other sub cellular compartment. The electrochemical potential arises from three factors: 1. the difference in the concentration of the ions on either side of the membrane, 2. the charge of the ions (for example Na+ , Ca++ , Cl− ), and 3. the difference in voltage between the two sides of the membrane (the transmembrane potential). 2 www.ck12.org Chapter 1. The Electrochemical Gradient - Advanced Cotransport of ions by symporters and antiporter carriers is commonly used to actively move ions across biological membranes. Transmembrane ATPases are often involved in maintaining ion gradients. The Na+/K+ ATPase uses ATP to build and maintain a sodium ion gradient and a potassium ion gradient. Proton Gradients and ATP synthase One particular ion gradient with biological significance is the proton (H+ ) gradient. This type of gradient is established through active transport involving proton pumps. The proton gradient is used during photosynthesis and cellular respiration to generate a chemiosmotic potential, or proton motive force. This potential energy is used for the synthesis of ATP by oxidative phosphorylation. The proton gradient can also be used to store energy for heat production and flagellar rotation. The energy held within the proton gradient can be used to synthesize ATP. ATP synthase is a transmembrane enzyme that provides energy for the cell to use by producing ATP. The protein has two distinct regions, F0 and F1 . The F0 domain is embedded within the membrane, while the F1 domain is above the membrane, inside the matrix of the mitochondria, or the stroma of the chloroplast. The F0 region is the proton pore, allowing hydrogen ions to diffuse across the membrane. The F1 region of the protein has ATP synthesis activity, catalyzing the formation of ATP from ADP and inorganic phosphate. Hence, ATP synthase is both an ion channel protein and enzyme. The synthesis reaction is driven by the proton flow, which forces the rotation of a part of the enzyme; the ATP synthase is a rotary mechanical motor. Bacteria may also have a version of this enzyme, where it, of course, is embedded in the cell membrane. During electron transport within the mitochondria (during cellular respiration) or chloroplast (during photosynthesis) (discussed in the Concept Metabolism (Advanced) concept), a proton gradient is formed when protons are pumped across the membrane by active transport. These hydrogen ions flow back across the membrane by facilitated diffusion through ATP synthase, releasing energy which is then used to convert ADP to ATP (by phosphorylation). Chemiosmosis is the diffusion of protons across the biological membrane through ATP synthase, due to a proton gradient that forms across the membrane during electron transport. Vocabulary • ATP synthase: Ion channel and enzyme complex; chemically bonds a phosphate group to ADP, producing ATP as H+ ions flow through the ion channel. • chemiosmosis: Process in cellular respiration or photosynthesis which produces ATP; uses the energy of hydrogen ions diffusing through ATP synthase. • chemiosmotic potential: A difference in concentration of hydrogen ions across a membrane within the mitochondrion or chloroplast; established using energy from an electron transport chain; also known as a chemiosmotic gradient. • electrochemical gradient: Difference across a membrane due to both a chemical force and an electrical force; drives the movement of ions across the membrane. • membrane potential: The voltage difference across a membrane; the basis for the conduction of nerve impulses along the cell membrane of neurons. • oxidative phosphorylation: A metabolic process that uses energy released by the oxidation of nutrients to produce adenosine triphosphate (ATP). 3 www.ck12.org • proton gradient: Gradient established from a higher concentration of protons on one side of a membrane compared to the other side of the membrane. • proton motive force: The storing of energy due to a combination of a proton gradient and a voltage gradient across a membrane. • resting potential: The membrane potential of a cell/neuron at rest; the membrane potential of an unstimulated neuron. • voltage: The difference in electrical potential energy of two points/areas; electrical potential energy that is caused by a separation of opposite charges. Summary • The voltage across a membrane is the membrane potential and the membrane potential of a cell at rest is the resting potential. • The electrochemical gradient is composed of a chemical force (the ions’ concentration gradient) and an electrical force (the effect of the membrane potential on the ions’ movement). • Chemiosmosis is the diffusion of protons across the biological membrane through ATP synthase, due to a proton gradient that forms across the membrane. Explore More Use this resource to answer the questions that follow. Gradients at http://www.youtube.com/watch?v=kQ_3mI0WY i0 MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/139344 1. Why does an electrochemical gradient form across a cell membrane? 2. Why are positive ions attracted to the inside of a cell? 3. How do ions flow in and out of a cell? Review 1. Define the electrochemical gradient. 2. Describe the role of ATP synthase. 3. What is chemiosmosis? References 1. Image copyright Alila Medical Media, 2014. Ionic basis of resting membrane potential . Used under license from Shutterstock.com 4